I beat Deus Ex in 25-30 hours(thats alot longer then most FPS), trying to get everything in the game, I've replayed it twice and found new stuff I didn't see before.

DX2's length is hard to determine, according to one of the developers, DX2 will have much more content, DX1 had 10,000 lines of text, DX2 has 30,000+. So it may be possible to beat it quicker then DX, but it has more packed in.